A fully coupled analysis of unsteady aerodynamics impact on vehicle dynamics during braking

Jakub Broniszewski,Janusz Piechna

doi:10.1080/19942060.2019.1616326

Jakub Broniszewski, Janusz Piechna

Open Access

https://doi.org/10.1080/19942060.2019.1616326

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Abstract

This study investigates the impact of unsteady aerodynamic loads on the behavior of car dynamics during braking maneuvers. In the analysis, two independent solvers were coupled to solve an existing fluid structure interaction (FSI) problem. Transient CFD analysis applying Fluent software was used to obtain realistic aerodynamic loads. A full car, nonlinear dynamic model with elastic suspension system was built in a dedicated multibody dynamic system MSC.Adams/Car. Both physics were integrated into one ecosystem via a block diagram environment for multidomain simulation (Matlab/Simulink). Methodology elements were validated against experimental results for a rectangular beam in cross-flow with vortex-induced vibrations. The final results were validated against a full-scale experiment on a real car. The solution of an untypical flow problem (decreasing car speed during a braking maneuver) required an untypical reference frame. The classic case, with an observer at rest (stationary model and moving air), was replaced with a moving observer. The whole domain was translocated during the analysis with a velocity which varied with time. The problem was further complicated by the presence of movable aerodynamic elements. The overset mesh technique was used to allow movement of the car body and active aerodynamic surfaces. The results obtained show the importance of aerodynamic effects on the braking process. It was shown that in the analyzed scenario, movable aerodynamic features can reduce the distance to stop by 6%. The complex interaction between the active aerodynamic surface and the car body with respect to load split is discussed as well. The methodology proposed to determine the behavior car dynamics with unsteady aerodynamic effects taken into account appears to be a significant improvement when compared with existing methodologies used for such an assessment.

Highlights

Unsteady aerodynamic effects are very common in normal life
Inclusion of the active aerodynamic surface reduces the distance to stop by 6% (6.7 m) and the braking time was shorter by 7.1% (0.37 s)
Those data clearly indicate that the downforce generated by a configuration with strong airfoil-car body interaction is much larger than the combined contribution of the isolated car body and the wing alone

Summary

Introduction

Unsteady aerodynamic effects are very common in normal life. Leaves on tree or a flag waving in the wind are examples from normal life. In the world of competition cars, the most famous examples of the consequences of rapid change in aerodynamic forces occurred during the Petit Le Mans race in 1998, 1999, and 2000. All of these races had cars taking-off and flipping in the air. The front wheels axis lost downforce in the wake of the preceding car Cases like these show how important the prediction of dynamics behavior could be when unsteady aerodynamic effects are taken into account. These are examples of fluid structure interaction (FSI) problems. A few factors were taken into account during the mesh size selection process:

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